Aqueous polymer dispersion

ABSTRACT

An aqueous polymer dispersion with a solids volume concentration of at least 50% by volume is obtainable by polymerizing monomers by the method of free radical aqueous emulsion polymerization with the addition of at least one fine and at least one coarse aqueous starting polymer dispersion.

This is a Division of application Ser. No. 08/229,631, filed on Apr. 19,1994, now U.S. Pat. No. 5,442,006, allowed; which is a DIV. of Ser. No.08/038,863 filed Mar. 29, 1993, now U.S. Pat. No. 5,352,720.

The present invention relates to a final aqueous polymer dispersionwhich has a solids volume concentration solids volume=solids massdivided by solids density) of at least 50% by volume and is obtainableby polymerizing at least one radical polymerizable monomer with theaddition of at least one aqueous starting dispersion I of a startingpolymer I and with the addition of at least one aqueous startingdispersion II of a starting polymer II in a polymerization vessel by themethod of free radical aqueous emulsion polymerization in the presenceof surface active substances and free radical polymerization initiatorswith the proviso that

a) the starting polymer I is present in the at least one aqueousstarting dispersion I in the form of dispersed starting polymerparticles I whose non-zero weight average particle diameter is equal toor less than 50 nm,

b) the mass of the at least one starting polymer I added relative to thetotal mass formed of the mass of the at least one radical polymerizablemonomer and of the mass of starting polymers I, II added in the form ofaqueous starting dispersions I, II is from 0.2 to 0.7%,

c) the starting polymer II is present in the at least one aqueousstarting dispersion II in the form of dispersed starting polymerparticles II of whose total mass not more than 60% have a non-zeroparticle diameter ≦200 nm and at least 40% have a particle diameter >200nm,

d) the mass of the at least one starting polymer II added relative tothe total mass formed of the mass of the at least one radicalpolymerizable monomer and of the mass of starting polymers I, II addedin the form of aqueous starting dispersions I, II is from 2 to 20%,

e) of the total amount of the at least one aqueous starting dispersion Ito be added the amount introduced into the polymerization vessel asinitial charge prior to the start of the free radical aqueous emulsionpolymerization is nil,

f) the total amount of the at least one aqueous starting dispersion IIto be added is introduced into the polymerization vessel as initialcharge prior to the start of the free radical emulsion polymerization,

g) of the total amount of the at least one monomer to by polymerized theproportion introduced into the polymerization vessel as initial chargeprior to the start of the free radical aqueous emulsion polymerizationis nil,

h) the total amount of the at least one monomer to be polymerized isadded to the polymerization vessel from the start of the free radicalaqueous emulsion polymerization in such a way that at any time of thisaddition the polymerization conversion of the total monomers alreadyadded previously to the polymerization vessel is at least 80 mol %,

i) at any time after the start of the free radical aqueous emulsionpolymerization the ratio V of the number of moles of the particles ofstarting polymer I already added previously to the polymerization vesselin the form of the at least one aqueous starting dispersion I to thenumber of moles of the amount already added previously to thepolymerization vessel of the at least one monomer to be radicalpolymerized, standardized for the ratio of the number of moles of thetotal starting polymer particles to be added in the form of the at leastone aqueous starting dispersion I to the number of moles of the totalamount of monomers to be radical polymerized, is within the rangefrom >0 to 10,

j) on completion of the addition of the total amount to be added of theat least one aqueous starting dispersion I additionally at most 20 mol %of the total amount of the at least one monomer to be radicalpolymerized is added to the polymerization vessel,

k) the at least one aqueous starting dispersion I to be added containsat least 10-50% by weight, based on its starting polymer I content, ofsurface active substances,

l) the at least one aqueous starting dispersion II to be added containsat least 1-5% by weight, based on its starting polymer II content, ofsurface active substances,

m) the final aqueous polymer dispersion contains from 1 % to 5% byweight of the surface active substances, based on the total mass formedof the mass of the at least one radical polymerizable monomer and of themasses of starting polymers I, II to be added in the form of aqueousstarting dispersions I, II,

n) the amount of surface active substances contained in thepolymerization vessel at any time from the start of the free radicalaqueous emulsion polymerization minus the minimum amounts mentionedunder k), l) of surface active substances introduced into thepolymerization vessel is less than 5% by weight, based on the monomersalready added previously to the polymerization vessel, and

o) the total amount used of free radical polymerization initiators isfrom 0.1 to 5% by weight, based on the total amount of the at least onemonomer to be radical polymerized, and is added to the polymerizationvessel in the course of the free radical aqueous emulsion polymerizationin such a way that the free radical aqueous emulsion polymerizationcontinues to a minimum polymerization conversion of the total amount ofthe at least one monomer to be polymerized of at least 90 mol %.

The present invention further relates to the process for preparing suchfinal aqueous polymer dispersions and to the use thereof as binders andas materials for preparing coatings and adhesive joints.

Aqueous polymer dispersions are systems comprising polymer particlesdispersed as disperse phase in an aqueous dispersion medium.

Polymer solutions form polymer films as the solvent evaporates. Aqueouspolymer dispersions behave the same way on evaporation of the aqueousdispersion medium, which is why aqueous polymer dispersions find varieduse as binders, for example for paints or for leather coatings.

Aqueous polymer dispersions having a high polymer content are ofparticular advantage in that, on the one hand, their relatively lowerproportion of aqueous dispersion medium reduces the energy required forevaporating it, for example for film formation or for preparing polymerpowders, and, on the other, the useful polymer can be stored andtransported using a relatively smaller amount of aqueous phase ascarrier medium.

However, there is a disadvantage in that, as the volume concentration ofthe polymer increases (U.S. Pat. No. 4,130,523), there are problems withthe preparation of aqueous polymer dispersions. For instance, the flowresistance (viscosity) increases and this increased viscosity makes itdifficult not only to remove the heat of reaction but also to processthe aqueous dispersion; secondly, there is an increasing tendency forthe dispersed polymer particles to aggregate for reasons ofthermodynamic stability. The resulting flocs [a) microflocs or specks;not normally removable by conventional filtration; b) macroflocs orcoagulum; normally removable by conventional filtration] interfere inparticular with the film forming of the aqueous polymer dispersions andare therefore generally undesirable.

According to studies about the flow resistance of aqueous polymerdispersions, those having a broad size distribution (polydispersity) ofthe dispersed polymer particles for the same solids content generallyhave a lower flow resistance than those with a narrow size distribution(which are in the extreme case monodispersed). Furthermore, coarseaqueous polymer dispersions have a lower flow resistance than fineaqueous polymer dispersions, given the same solids content.

EP-A-129 699 discloses a process for preparing an aqueous polymerdispersion wherein unsaturated monomers are polymerized in aconventional manner in a polymerization vessel by the method of freeradical aqueous emulsion polymerization with the addition of an aqueousdispersion of a starting polymer such that the addition of the aqueousdispersion of the starting polymer must be concluded before 40% byweight of the total monomers to be polymerized have copolymerized andmust not start before the average particle size of the emulsion polymerformed in the course of the polymerization of the monomers is twice thatof the aqueous dispersion of the starting polymer. In fact, the aqueousdispersion of the starting polymer is preferably not added over aprolonged period but all at once.

The disadvantages of the aqueous polymer dispersions thus obtainable arethat their flow resistance is not fully satisfactory above a solidsvolume concentration of 50% by volume and that, according to theembodiment examples, the solids volume concentration is limited tovalues below 65% by volume.

U.S. Pat. No. 4,130,523 concerns a process for preparing aqueous polymerdispersions wherein aqueous polymer dispersion already formed in thecourse of the polymerization process is continuously removed from thereaction zone, stored and later reintroduced into the reaction zone as akind of starting polymer dispersion. A disadvantage of this process isthat it is unsuitable for industrial implementation.

U.S. Pat. No. 3,424,706 concerns a process for preparing aqueousdispersions of polymers containing at least 70-97% by weight ofvinylidene chloride as copolymerized units, wherein the polymerizationof the monomers is effected with the addition of an aqueous dispersionof a starting polymer. The said reference teaches inter alia mixing themonomers to be polymerized and the aqueous dispersion of the startingpolymer with one another and adding this mixture to the initial chargecomprising part of the polymerization batch.

The disadvantage with this process is that it is restricted to monomermixtures consisting chiefly of vinylidene chloride. Moreover, accordingto the illustrative embodiments, the aqueous polymer dispersionsobtainable by this process are unsatisfactory not only as regards theflow resistance above a solids volume concentration of 50% by volume butalso as regards the upper limit for the solids volume concentrationattainable in a still satisfactorily flowable state.

It is an object of the present invention to make available aqueouspolymer dispersions that are obtainable in a simple, industriallysuitable, reproducible manner not restricted to specific monomers withan increased solids volume concentration but a reduced flow resistanceand reduced floc content.

We have found that this object is achieved by the final aqueous polymerdispersions defined at the beginning.

Remarkably, the subject-matter of the invention is not restricted to thefree radical aqueous emulsion polymerization of monomer mixturescomposed chiefly or exclusively of vinyl and/or vinylidene halides,despite the generally known fact that the development of the dispersephase in the case of monomers other than vinyl and/or vinylidene halidesis a significantly more complex phenomenon.

Suitable radical polymerizable monomers for the process of the inventionare therefore in particular, inter alia, monoethylenically unsaturatedmonomers such as olefins, for example ethylene, aromatic vinyl monomerssuch as styrene, α-methylstyrene, o-chlorostyrene or vinyltoluenes,vinyl and vinylidene halides such as vinyl and vinylidene chloride,esters of vinyl alcohol and monocarboxylic acids having from 1 to 18carbon atoms, such as vinyl acetate, vinyl propionate, vinyl-n-butyrate,vinyl laurate and vinyl stearate, esters of α,β-monoethylenicallyunsaturated mono- and dicarboxylic acids preferably of from 3 to 6carbon atoms, such as, in particular, acrylic acid, methacrylic acid,maleic acid, fumaric acid and itaconic acid, with alkanols in general offrom 1 to 12, preferably of from 1 to 8, in particular of from 1 to 4,carbon atoms, such as, in particular, methyl, ethyl, n-butyl, isobutyland 2-ethylhexyl acrylate and methacrylate, dimethyl maleate or n-butylmaleate, nitriles of α,β-monoethylenically unsaturated carboxylic acidssuch as acrylonitrile and also C₄₋₈ -conjugated dienes such as1,3-butadiene and isoprene. The monomers mentioned generally form theprincipal monomers which, based on the total amount of the monomers tobe polymerized by the method of free radical aqueous emulsionpolymerization, normally account for a proportion of more than 50% byweight. Monomers which polymerized by themselves normally formhomopolymers that possess enhanced water solubility are normallyincluded in the polymer only as modifying monomers, in amounts, based onthe total amount of monomers to be polymerized, of less than 50% byweight, in general from 0.5 to 20, preferably from 1 to 10, % by weight.

Examples of monomers of this type are α,β-monoethylenically unsaturatedmono- and dicarboxylic acids of from 3 to 6 carbon atoms and amidesthereof, e.g. acrylic acid, methacrylic acid, maleic acid, fumaric acid,itaconic acid, acrylamide and methacrylamide, also vinylsulfonic acidand water-soluble salts thereof, and also N-vinylpyrrolidone. Monomerswhich customarily enhance the internal strength of the films formed fromthe final aqueous polymer dispersion are in general likewise included inthe polymer only in minor amounts, usually from 0.51 to 10% by weight,based on the total amount of monomers to be polymerized. Monomers ofthis type normally have an epoxy, hydroxyl, N-methylol, carbonyl or atleast two nonconjugated ethylenically unsaturated double bonds. Examplesthereof are N-alkylolamides of α,β-monoethylenically unsaturatedcarboxylic acids of from 3 to 10 carbon atoms or esters thereof withalcohols of from 1 to 4 carbon atoms, of which N-methylolacrylamide andN-methylolmethacrylamide are very particularly preferred, divinylmonomers, divinylidene monomers and also dialkenyl monomers.Particularly suitable instances of these are the diesters of dihydricalcohols with α,β-monoethylenically unsaturated monocarboxylic acids, ofwhich in turn acrylic and methacrylic acid are preferred. Examples ofsuch monomers having two nonconjugated ethylenically unsaturated doublebonds are alkylene glycol diacrylates and dimethacrylates such asethylene glycol diacrylate, 1,3-butylene glycol diacrylate, 1,4-butyleneglycol diacryalte, propylene glycol diacrylate, divinylbenzene, vinylmethacrylate, vinyl acrylate, allyl methacrylate, allyl acrylate,diallyl maleate, diallyl fumarate, methylenebisacrylamide,cyclopentadienyl acrylate and triallyl cyanurate. In this connection ofparticular importance are also C₁ -C_(w) -hydroxyalkyl methacrylate andacrylate, such as n-hydroxyethyl, n-hydroxypropyl or n-hydroxybutylacrylate and methacrylate, and also compounds such asdiacetoneacrylamide and acetylacetoxyethyl acrylate or methacrylate. Aswell as monomers having unsaturated double bonds it is possible forminor amounts, customarily from 0.01 to 2% by weight, based on themonomers to be polymerized, of molecular weight regulators, such astert-dodecyl mercaptan and 3-mercaptopropyltrimethoxysilane to beincluded in the polymer. It is preferable to add such substances to thepolymerization zone mixed with the monomers to be polymerized.

Suitable surface active substances include not only the protectivecolloids customarily used for carrying out free radical aqueous emulsionpolymerizations but also emulsifiers. Examples of suitable protectivecolloids are polyvinyl alcohols, cellulose derivatives andvinylpyrrolidone-containing copolymers. A detailed description offurther suitable protective colloids may be found in Houben-Weyl,Methoden der organischen Chemie, Volume XIV/1, Makromolekulare Stoffe,Georg Thieme Verlag, Stuttgart, 1961, pages 411 to 420. It is of coursealso possible to use mixtures of emulsifiers and/or protective colloids.Preferably, the surface active substances used are exclusivelyemulsifiers whose relative molecular weights are customarily below 1000,in contradistinction to the protective colloids. They can be anionic,cationic or nonionic in nature. Of course, if mixtures of surface activesubstances are used, the individual components must be compatible withone another, which can be verified beforehand by means of a fewpreliminary experiments if there is any doubt. In general, anionicemulsifiers are compatible with one another and with nonionicemulsifiers. The same is true of cationic emulsifiers, while anionic andcationic emulsifiers are usually incompatible with one another. Examplesof customary emulsifiers are ethoxylated mono-, di- and trialkylphenols(EO degree: 3-50, alkyl radical: C₄ -C₉), ethoxylated fatty alcohols (EOdegree: 3-50, alkyl radical: C₈ -C₃₆), and also alkali metal andammonium salts of alkyl sulfates (alkyl radical: C₈ -C₁₂), of sulfuricmonoesters of ethoxylated alkanols (EO degree: 4-30, alkyl radical: C₁₂-C₁₈), and ethoxylated alkylphenols (EO degree: 3-50, alkyl radical: C₄-C₉), of alkylsulfonic acids (alkyl radical: C₁₂ -C₁₈) and ofalkylarylsulfonic acids (alkyl radical: C₉ -C₁₈). Further suitableemulsifiers may be found in Houben-Weyl, Methoden der organischenChemie, Volume XIV/1, Makromolekulare Stoffe, Georg Thieme Verlag,Stuttgart, 1961, pages 192 to 208.

Particularly suitable surface active substances are compounds of thegeneral formula I ##STR1## where R¹ and R² are each hydrogen or C₄ -C₂₄-alkyl but are not both hydrogen, and X and Y are each an alkali metalor ammonium ion. In the formula I, R¹ and R² are each preferably linearor branched alkyl radicals of from 6 to 18 carbon atoms or hydrogen, inparticular of 6, 12 and 16 carbon atoms, but R¹ and R² must not both behydrogen. X and Y are each preferably sodium, potassium or ammoniumions, of which sodium is particularly preferred. Of particular advantageare compounds I in which X and Y are each sodium, R¹ is a branched alkylradical of 12 carbon atoms and R² is hydrogen or R¹. It is common toemploy technical grade mixtures containing from 50 to 90% by weight ofthe monoalkylated product, for example Dowfax® 2A1 (trademark of the DowChemical Company). In the process of the invention compounds I arepreferably used as the sole surface active substances and particularlypreferably in mixture with ethoxylated fatty alcohols (EO degree: 3-50,alkyl radical: C₈ -C₃₆). Compounds I are generally known, for examplefrom U.S. Pat. No. 4,269,749, and are commercially available. It isadvantageous for the final aqueous polymer dispersion of the inventionto contain from 1 to 3% by weight of surface active substances, based onthe mass of the final polymer.

Suitable free radical polymerization initiators are all those which arecapable of initiating a free radical aqueous emulsion polymerization.This includes not only peroxides, for example alkali metalperoxodisulfates, but also azo compounds. Preference is given to usingcombined systems composed of at least one organic reducing agent and atleast one peroxide and/or hydroperoxide, e.g. tert-butyl hydroperoxideand the sodium salt of hydroxymethanesulfinic acid or hydrogen peroxideand ascorbic acid, and very particular preference is given to combinedsystems which in addition contain a small amount of a metal compoundthat is soluble in the polymerization medium and whose metalliccomponent can exist in a plurality of valence states, for exampleascorbic acid/iron(II) sulfate/hydrogen peroxide, although in the caseof ascorbic acid it is also common to employ the sodium salt ofhydroxymethanesulfinic acid, sodium sulfite, sodium hydrogensulfite orsodium disulfite and instead of hydrogen peroxide it is common to employtert-butyl hydroperoxide or alkali metal peroxodisulfates and/orammonium peroxodisulfate. Instead of a water-soluble iron(II) salt it iscommon to employ a combination of water-soluble Fe/V salts. The amountof free radical initiator system used is preferably from 0.1 to 2% byweight, based on the total amount of monomers to be polymerized.

The manner of addition of the free radical initiator system to thepolymerization vessel in the course of the free radical aqueous emulsionpolymerization of the invention is rather of minor importance for thesuccess of the process of the invention. The initiator system can notonly be introduced into the polymerization vessel in its entirety aspart of the initial charge but also be added continuously or stepwise inthe course of the free radical aqueous emulsion polymerization at therate of its consumption. The choice in a particular case depends in theusual fashion not only on the chemical nature of the initiator systembut also on the polymerization temperature.

The polymerization pressure and the polymerization temperature arelikewise of rather minor importance. In general, the temperatureemployed will be between room temperature and 100° C., preferably withinthe range from 50° to 95° C. The employment of superatmospheric orreduced pressure is possible, so that the polymerization temperature mayalso exceed 100° C. and may in fact be as high as 130° C. Volatilemonomers such as ethylene, butadiene or vinyl chloride are preferablypolymerized under superatmospheric pressure. To control the pH of thepolymerization medium ammonia, for example, may be added during the freeradical aqueous emulsion polymerization of the invention.

The radical polymerizable monomers mentioned by way of example assuitable for the free radical aqueous emulsion polymerization of theinvention are suitable not only for use as constituents of the monomermixture to be polymerized, but also, in the same way as thepolymerization initiators, molecular weight regulators and pH regulatorsrecommended for the aqueous emulsion polymerization of the invention,for use as constituents of the starting polymers I, II and of thestarting dispersions containing these, although the monomer, regulatorand initiator composition for the aqueous starting dispersions I, II canbe not only congruent with but also different from the monomer mixtureto be polymerized according to the invention. This applies mutatismutandis necessarily also to the surface active substances to be usedfor preparing the aqueous starting dispersions I, II, since in theprocess of the invention, relative to the amounts of starting polymers Iand II, at least 10-50 (starting dispersion I) and at least 1-5% byweight (starting dispersion II) of the surface active substances finallycontained in the final aqueous polymer dispersion of the invention arealready part of the aqueous starting dispersions I, II to be added.These amounts of surface active substances are in general the minimumamounts required for preparing stable aqueous starting dispersions I,II.

Aqueous starting dispersions I to be added according to the invention ofa starting polymer I in which the starting polymer I is present in theform of dispersed starting polymer particles having a non-zero weightaverage particle diameter d_(w) of less than or equal to 50 nm,preferably from 25 to 45 nm, are obtainable in a conventional manner.Appropriate teaching may be found for example in Houben-Weyl, Methodender organischen Chemie, Voluem E 20, part I, Makromolekulare Stoffe,Georg Thieme Verlag, Stuttgart, 1987, pages 248 to 268. If the solidscontent is to be a preferred 20-40% by weight, the starting dispersion Iare obtainable in a particularly simple manner, for example by mixingtogether the aqueous phase, the monomers, the free radical initiators(normally from 0.1 to 5% by weight, based on the amount of startingmonomers to be polymerized), and surface active substances (customarilyfrom 10 to 50% by weight, based on the starting monomers to bepolymerized) at a low temperature and heating the mixture to thepolymerization temperature and polymerizing it (the particle diameterd_(w) generally decreases with increasing surface active substance). Inanother version, the initial charge comprises essentially all thecomponents, but the polymerization initiator is added continuously afterthe charge has been heated to the reaction temperature and while it ismaintained at that temperature. As regards the polymerizationtemperature and pressure the earlier statements about the process of theinvention apply. Aqueous starting dispersion I which have a broadparticle size distribution are preferred. Preference is also given tothose starting dispersions I which contain as surface active substancesexclusively those of the general formula I.

Starting dispersions II are likewise obtainable in a manner known per seto one skilled in the art. Essentially, they can be prepared in the sameway as the starting dispersions I except for the difference that theamount of surface active substances used relative to the monomers to bepolymerized is in general merely 1-5% by weight. Preferred startingdispersions II are those which have a broad particle size distribution.Particular preference is given to aqueous polymer dispersions claimed inthe main claim of DE-A . . . (O.Z. 0050/43182), which in general havethe following particle size distribution:

5-25% by weight of polymer≦200 nm

15-40% by weight of polymer≦400 nm

35-75% by weight of polymer≦600 nm

45-90% by weight of polymer≦800 nm

100% by weight of polymer≦1600 nm.

Other suitable starting dispersions II are the aqueous polymerdispersions claimed in claim 6 of De-A . . . (O.Z. 0050/43185).

If starting dispersion polymers II having a narrow particle sizedistribution are used, the amount thereof used in the process of theinvention is preferably from 2 to 10, particularly preferably from 2 to5, % by weight, based on the total mass formed of the mass of the atleast one radical polymerizable monomer and the masses of startingpolymers I, II added in the form of aqueous starting dispersions I, II.

Very particular preference is given to using as starting dispersions IIfinal polymer dispersions which in turn have been obtained by theprocess disclosed in the present invention. At least part of the finalaqueous polymer dispersion obtainable therein can in turn be used againas starting dispersion II, and so on. If as part of such a cyclicalprocedure otherwise constant amounts and starting materials areemployed, the remarkable result is that, generally independently of thedispersity of the particle size distribution of the original startingdispersion II and at the latest after the third cycle, a final polymerdispersion whose application properties (flowability) will in generalnot change within the margin of error of measurement in the course offurther cycles. This cyclical procedure offers in particular an elegantway of producing the final aqueous polymer dispersions of the inventionin succession in essentially constant quality in a simple, industriallyimplementable manner by using a constant proportion of the final polymerdispersion obtained as starting dispersion II in the subsequent cycle,with or without dilution, while keeping other process parametersconstant. Industrial implementation of such a cyclical procedure isparticularly simple when at the end of a cycle the final polymerdispersion in the polymerization vessel is only part-discharged (theproportion being discharged being customarily 80-90% by weight) and theremainder is used with or without dilution as starting dispersion II inthe next cycle, and so on. Under constant conditions three cycles willin general, as mentioned, result in an essentially steady state, ie. anend product of essentially consistent quality (flowability). Normally,the starting dispersions II are used in the process of the inventionwith a solids content of from 40 to 60% by weight. In general, theparticle diameters of starting dispersion II are ≦2000 nm. Of course,the aqueous starting dispersion II to be used according to the inventioncan also be prepared immediately beforehand in the same polymerizationvessel in which the actual process of the invention is to be carriedout, whether for example by mixing different polymer dispersions in thepolymerization vessel (a simple way of preparing an aqueous startingdispersion II having a broad particle size distribution) and/or bycarrying our a free radical aqueous emulsion polymerization therein.

In the latter case, the two emulsion polymerization processes can mergesmoothly, in particular when the aqueous starting dispersion II isprepared employing the stream addition process, in which part of thepolymerization batch is introduced as initial charge and heated to thepolymerization temperature and the remainder is then added to theongoing polymerization in a stream or spatially separate streams. Ofcourse, in the course of the aqueous free radical emulsionpolymerization of the invention it is possible for there to be a changein the composition not only of the monomers to be added but also of theaqueous starting dispersion I to be added. Furthermore, the additionscan take place not only stepwise but also continuously or by thegradient method. Preferably, the addition of the monomers to bepolymerized takes place relative to the addition of the at least oneaqueous starting dispersion I in such a way that V is within therange >0 to 5 at any time after the start of the aqueous emulsionpolymerization of the invention. Here a continuous mode of addition ispreferred not only for the monomers to be polymerized but also for theat least one aqueous starting dispersions I. It is particularlyadvantageous for the monomers to be polymerized and the at least oneaqueous starting dispersion I to be added to be added continuously andsynchronously starting at the same time and preferably finishing at thesame time. Moreover, the monomers to be polymerized are advantageouslyadded to the reaction zone in such a way that at any time of thisaddition the polymerization conversion of the monomers already addedpreviously to the polymerization vessel is at least 90 mol %,particularly preferably at least 98 mol %. The monomers to bepolymerized can be added not only by themselves but also preemulsifiedin an aqueous phase. If final aqueous polymer dispersions having aparticularly high solids volume concentration are to be produced, themonomers to be polymerized are preferably added by themselves.Advantageously, prior to the start of the free radical emulsionpolymerization of the invention the polymerization vessel contains onlysome of the free radical polymerization initiators, whereas theremaining free radical polymerization initiators are in general added tothe polymerization zone at the start of the free radical emulsionpolymerization of the invention via a spatially separate stream, whichaddition preferably takes place synchronously with the addition of themonomers to be polymerized, but in general will take longer than thelatter addition. For this reason the initiation of the aqueous freeradical emulsion polymerization of the invention is in general effectedby adding monomers to be polymerized to an initial charge that alreadycontains polymerization initiators, after heating to the polymerizationtemperature, or by accompanying the addition of these monomers to bepolymerized by the addition of initiator, if the initial charge heatedto the polymerization temperature does not contain any initiator. If,from the start of the free radical aqueous emulsion polymerization ofthe invention, surface active substances are added to the polymerizationvessel in an amount exceeding the minimum amount of 10-50% by weight,based on starting polymer I, required in the at least one aqueousstarting dispersion I, this addition is advantageously effected in sucha way that at any time from the start of the free radical aqueousemulsion polymerization of the invention the amount of surface activesubstances contained in the polymerization vessel minus the minimumamounts of surface active substances introduced into the polymerizationvessel as minimum amounts mentioned under k), l) is less than 3% byweight, based on the monomers already added to the polymerizationvessel. Preferably the addition takes place synchronously with themonomer addition.

On completion of the actual polymerization process of the invention themixture is preferably stirred for some additional hours while thepolymerization temperature is maintained. This may be followed bycustomary measures for residual monomer removal, for setting a differentpH or other methods of post-stabilization, including the subsequentaddition of surface active substances. Of course, the various possible,generally spatially separate, feed streams can be mixed with one anotherimmediately before entry into the polymerization vessel.

Preferred classes of final polymers are those composed

to an extent of from 70 to 100% by weight of esters of acrylic and/ormethacrylic acid with alkanols of from 1 to 12 carbon atoms and/orstyrene

or

to an extent of from 70 to 100% by weight of styrene and/or butadiene

or

to an extent of from 70 to 100% by weight of vinyl chloride and/orvinylidene chloride,

of which the class of the acrylates is particularly preferred andpreferably comprises the following monomer compositions:

70-99% by weight of at least one ester of acrylic and/or methacrylicacid with alkanols of from 1 to 8 carbon atoms,

1-5% by weight of acrylic acid, methacrylic acid or a mixture thereof,and

0-25% by weight of vinyl acetate, styrene or a mixture thereof.

The free radical aqueous emulsion polymerization of the invention makesit possible to produce in a simple manner final aqueous polymerdispersions which have a very wide particle size distribution which canbe characterized as follows:

5-30% by weight of the final polymer≦200 nm

20-55% by weight of the final polymer≦300 nm

45-70% by weight of the final polymer≦400 nm

60-85% by weight of the final polymer≦600 nm

75-95% by weight of the final polymer≦800 nm

100% by weight of the final polymer ≦2000 nm.

It is presumably this specific particle size distribution which isresponsible for the reduced flow resistance of the final aqueous polymerdispersions of the invention, which normally have Newtonian flowcharacteristics. The particle size distribution was determined in ananalytical ultracentrifuge using the coupling PSD technique (see W.Machtle, Angewandte Makromolekulare Chemie 162 (1988), 35-42 (No.2735)); the particle size determinations on the aqueous startingdispersions having a less wide particle size distribution were likewisecarried out using the analytical ultracentrifuge (cf. W. Machtle,Makromolekulare Chemie 185 (1984), 1025-1039). Below a solids volumeconcentration of 50% by volume the effect of the particle sizedistribution on the flow resistance decreases progressively. The finalaqueous polymer dispersions of the invention are generally obtained asdescribed in an industrially readily implementable manner with solidsvolume concentrations of up to 75% by volume with fully satisfactoryreproducibility and no flocs.

The final aqueous polymer dispersions of the invention show theiradvantageous properties particularly markedly at solids volumeconcentrations above 65% by volume, which is why such final polymerdispersions are preferred. They are generally suitable for use asbinders and as materials for preparing coatings and adhesive joints, forwhich purpose they may have additionally mixed into them in aconventional manner assistants such as film forming aids, fillers orplasticizers.

EXAMPLES Example 1

Preparation of starting dispersion I

SIa): a mixture of 65.4 kg of water,

25 kg of n-butyl acrylate and

22.25 kg of a 45% strength by weight aqueous solution of the surfaceactive substance corresponding to Dowfax 2A1

was admixed at room temperature first with a solution of 0.0625 kg ofascorbic acid and 0.005 kg of iron(II) sulfate in 2 kg of water and thenin the course of 3 minutes with a mixture of 2 kg of water and 0.65 kgof a 30% strength by weight aqueous hydrogen peroxide solution. Thetemperature was then maintained at 52° C. for 1 h. Thereafter themixture was cooled down to room temperature and following the additionof 0.05 kg of ascorbic acid in 1 kg of water additionally stirred for 2h.

The resulting aqueous starting dispersion Ia) had a solids volumeconcentration of 28.5% by volume (solids content=30.2% by weight) and aweight average particle diameter d_(w) of 32 nm.

SIb): a mixture of the composition

51.12 kg of water,

0.40 kg of a 30% strength by weight aqueous H₂ O₂ solution,

20 kg of n-butyl acrylate, and

18.80 kg of a 45% strength by weight aqueous solution of the surfaceactive substance corresponding to Dowfax 2A1

was admixed at 25° C. in one portion with 50% by weight of the feedstream defined hereinafter and then heated to 50° C. Then the remainderof the stream was added continuously over 30 min while the 50° C. weremaintained. This was followed by a further hour of stirring at 50° C.

    ______________________________________                                        Feed stream:                                                                  ______________________________________                                                 4    kg of water                                                              0.20 kg of ascorbic acid                                                      0.001                                                                              kg of iron (II) sulfate                                         ______________________________________                                    

The resulting aqueous starting dispersion Ib) had a solids volumeconcentration of 28.5% by volume (solids content=30.1% by weight) and aweight average particle diameter d_(w) of 44 nm.

Example 2

Preparation of starting dispersions II

    ______________________________________                                        SIIa): A mixture of the composition                                           ______________________________________                                                  546  g of water                                                               1.35 g of ascorbic acid                                                       60   g of stream II                                                 ______________________________________                                    

was heated to 85° C. and admixed continuously, starting at the sametime, with the remainder of stream II (in the course of 3.5 h) and withstream I (in the course of 3 h) while the 85° C. were maintained. Oncompletion of the addition of the remainder of stream II the mixture wasadditionally stirred at 85° C. for 1 h.

    ______________________________________                                        Stream I:                                                                     573     g of water                                                            1764    g of n-butyl acrylate                                                 36      g of methacrylic acid                                                 180     g of a 20% strength by weight aqueous solution                                of the surface active substance corresponding to                              Dowfax 2A1                                                            0.45    g of ascorbic acid                                                    Stream II:                                                                    600     g of water                                                            9       g of sodium peroxodisulfate                                           ______________________________________                                    

The resulting aqueous starting dispersion IIa) had a solids volumeconcentration of 45.7% by volume (solids content=49.3% by weight) and anessentially monodisperse particle size distribution with a particlediameter d_(w) of 294 nm.

SIIb): a mixture of the composition

11.67 g of aqueous starting dispersion SIa) of Example 1,

4.375 g of a 20% strength by weight aqueous solution of the surfaceactive substance corresponding to Dowfax 2A1,

17.50 g of a 30% strength by weight aqueous H₂ O₂ solution,

40.5 g of stream II, and

600 g of water

was heated to 60° C. and while this temperature was maintainedcontinuously admixed, starting at the same time, with the remainder ofstream II and with stream I as per the following schedules:

    ______________________________________                                        Stream I:     5% by weight within 30 min, then                                             95% by weight within 150 min.                                    Stream II:   90% by weight within 210 min.                                    ______________________________________                                    

(% by weight based on the respective total amount)

Stream III was added continuously within 120 min starting 30 min fromthe start of the addition of stream I.

    ______________________________________                                        Stream I:                                                                     1645    g of n-butyl acrylate                                                 70      g of ethyl acrylate                                                   35      g of methacrylic acid                                                 70      g of a 20% strength by weight aqueous solution                                of the surface active substance corresponding                                 to Dowfax 2A1                                                         175     g of a 20% strength by weight aqueous solution                                of ethoxylated fatty alcohols (C.sub.16 -C.sub.18,                            EO degree: 30)                                                        0.875   g of 3-mercaptopropyltrimethoxy-                                              silane                                                                391     g of water                                                            Stream II:                                                                    5.25    g of ascorbic acid                                                    0.175   g of Iron (II) sulfate                                                400     g of water                                                            Stream III:                                                                   58.33   g of aqueous starting dispersion                                              SIa) of Example 1                                                     13.13   g of a 20% strength by weight aqueous solution                                of the surface active substance corresponding                                 to Dowfax 2A1                                                         100     g of water                                                            ______________________________________                                    

The resulting aqueous starting dispersion IIb) had a solids volumeconcentration of 45.6% by volume (solids content=50.3% by weight) andthe following particle size distribution:

15% by weight of starting polymer IIb≦150 nm

58% by weight of starting polymer IIb≦200 nm

88% by weight of starting polymer IIb≦300 nm

100% by weight of starting polymer IIb≦400 nm

SIIc): a mixture of the composition

22.63 g of stream II,

42.00 g of stream III, and

300 g of water

was heated to 85° C. and while this temperature was maintained admixedcontinuously, starting at the same time, with the remainder of stream IIand with stream I as per the following schedules:

    ______________________________________                                        Stream I:     4% by weight within 30 min, then                                             96% by weight within 180 min                                     Stream III:  80% by weight within 240 min                                     ______________________________________                                    

(% by weight based on the respective total amount)

The remainder of stream II was added continuously within 180 minstarting 30 min from the start of the addition of stream I.

    ______________________________________                                        Stream I:                                                                     1710   g of n-butyl acrylate                                                  250    g of methyl methacrylate                                               40     g of methacrylic acid                                                  1      g of tert-dodecylmercaptan                                             50     g of a 20% strength by weight aqueous solution                                of ethoxylated fatty alcohols (C.sub.16 -C.sub.18,                            EO degree: 18)                                                         100    g of a 20% strength by weight aqueous solution                                of the surface active substance corresponding to                              Dowfax 2A1                                                             1      g of 3-mercaptopropyltrimethoxy-                                              silane                                                                 470    g of water                                                             Stream II:                                                                    13.33  g of aqueous starting dispersion SIa)                                         of Example 1                                                           5      g of a 20% strength by weight aqueous solution                                of the surface active substance corresponding to                              Dowfax 2A1                                                             8      g of a 25% strength by weight aqueous ammonia                                 solution                                                               200    g of water                                                             Stream III:                                                                   10.00  g of sodiun peroxodisulfate                                            200    g of water                                                             ______________________________________                                    

The resulting aqueous starting dispersion IIc) had a solids volumeconcentration of 56.1% by volume (solids content=61.1% by weight) andthe following particle size distribution:

4% by weight of starting polymer IIc)≦200 nm

6% by weight of starting polymer IIc)≦300 nm

60% by weight of starting polymer IIc)≦400 nm

100% by weight of starting polymer IIc)≦600 nm

Example 3

Preparation of final aqueous polymer dispersions according to theinvention

Fa): 1000 g of SIIa) of Example 2 were heated to 85° C. and thencontinuously admixed at that temperature with streams I, II and III inthe course of 4 h, starting at the same time. Subsequently the reactionmixture was maintained at 85° C. for a further 1 h.

    ______________________________________                                        Stream I:                                                                     1960   g of n-butyl acrylate                                                  40     g of methacrylic acid                                                  Stream II:                                                                    25     g of aqueous starting dispersion                                              SIa) of Example 1                                                      170    g of water                                                             20     g of a 10% strength by weight aqueous                                         ammonia solution                                                       150    g of a 20% strength by weight aqueous solution                                of the surface active substance corresponding to                              Dowfax 2A1                                                             Stream III:                                                                   160    g of water                                                             10     g of sodium peroxodisulfate                                            ______________________________________                                    

Fb): As for Fa), except that the initial charge contained only 500 g ofSIIa) and that in stream II the 170 g of water were replaced with 320 gof water.

Fc): As for Fa), except that the initial charge contained only 250 g ofSIIa) and that in stream II the 170 g of water were replaced with 400 gof water.

Fd): As for Fa), except that the initial charge contained 500 g of Fa)of Example 3 instead of 1000 g of water SIIa) and that in stream II the170 g of water were replaced with 470 g of water.

Fe): As for Fd), except that the initial charge contained Fd) instead ofFa).

Ff): As for Fe), except that the initial charge contained Fe) instead ofFd).

Fg): As for Ff), except that the initial charge contained Ff) instead ofFe).

The final polymer dispersions thus obtained are characterized in Table 1(SC=solids content in % by weight, SV=solids volume concentration in %by volume, η=dynamic viscosity at 23° C. and a shear gradient of 487 s⁻¹as per DIN 53 019 in mPa·s). The particle size distribution is reportedin each case in terms of the proportion (%) by weight of the finalpolymer whose particle diameter is ≦X nm, where X is an element of theset {200, 300, 400, 600, 800, 1000,

                                      TABLE 1                                     __________________________________________________________________________    SC     SV η                                                                             200                                                                              300 400                                                                              600                                                                              800 1000                                                                             1800                                        __________________________________________________________________________    Fa) 71.4                                                                             66.1                                                                             626 13 15  16 98 100                                                Fb) 71.0                                                                             65.7                                                                             330 15 17  17 95 100                                                Fc) 72.2                                                                             66.8                                                                             290 33 35  35 55 100                                                Fd) 72.1                                                                             66.8                                                                             243 18 44  45 45 92  100                                            Fe) 71.0                                                                             65.7                                                                             303 15 28  50 75 75  77 100                                         Ff) 72.3                                                                             66.9                                                                             291 18 35  48 70 88  90 100                                         Fg) 72.1                                                                             66.8                                                                             394 15 30  54 75 85  95 100                                         Fh): A mixture of                                                                               500.8 g                                                                           of SIIb) (Example 2)                                                      17.5 g                                                                            of 30% strength by                                           Initiator-containing                                                                           weight aqueous H.sub.2 O.sub.2                               starting         solution                                                     dispersion SIId)                                                                           14.67 g                                                                           of stream II                                                              100 g                                                                             of water                                                __________________________________________________________________________

was heated to 60° C. and then admixed in one portion with 10% by weightof stream III. Then the continuous addition was commenced at the sametime of the remainders of streams II and III and also of stream I(stream I and the remainder of stream II being added within 3 h, theremainder of stream III within 4 h) while the reaction temperature wasraised to 70° C. and left at that level to the completion of theaddition of stream III.

    ______________________________________                                        Stream I:                                                                     1645   g of n-butyl acrylate                                                  70     g of ethyl acrylate                                                    35     g of methacrylic acid                                                  0.875  g of 3-mercaptopropyltrimethoxy-                                              silane                                                                 175    g of a 20% strength by weight aqueous solution                                of ethoxylated fatty alcohols (C.sub.16 -C.sub.18,                            EO degree: 30)                                                         70     g of a 20% strength by weight aqueous solution                                of the surface active substance corresponding to                              Dowfax 2A1                                                             275    g of water                                                             Stream II:                                                                    17.5   g of a 20% strength by weight aqueous solution                                of the surface active substance corresponding to                              Dowfax 2A1                                                             29.17  g of SIa) (Example 1)                                                  100    g of water                                                             Stream III:                                                                   5.25   g of ascorbic acid                                                     0.175  g of iron (II) sulfate                                                 100    g of water                                                             ______________________________________                                    

Fi): As for Fh), except that the initial charge contained 500.8 g ofFh), (Example 3) instead of 500.8 g of SIIb) and that in stream I the275 g of water were replaced with 388 g of water.

Fj): As for Fi), except that the initial charge contained Fi) instead ofFh).

Fk): As for Fj), except that the initial charge contained Fj) instead ofFi).

Fl): As for Fh), except that the initial charge contained 250.25 g ofSIIb) instead of 500.8 g of SIIb) and that in stream I the 275 g ofwater were replaced with 335 g of water.

Fm): As for Fh), except that the initial charge contained 500.8 g of Fh)instead of 500.8 g of SIIb), that in stream I the 275 g of water werereplaced with 320 g of water, that in stream II the 100 g of water werereplaced with 200 g of water and that in stream III the 100 g of waterwere replaced with 200 g of water.

Fn): As for Fh), except that the streams I, II, III and the initialcharge had the following compositions:

    ______________________________________                                        Initial charge:                                                               256.94                                                                              g of Fm) (Ex. 3)                                                        9     g of a 30% strength by weight                                                                          Initiator-containing                                 H.sub.2 SO.sub.2 solution                                                                              starting dispersion                            12.4  g of stream II           SIIe)                                          50    g of water                                                              Stream I:                                                                     666   g of n-butyl acrylate                                                   36    g of ethyl acrylate                                                     18    g of methacrylic acid                                                   180   g of vinyl acetate                                                      90    g of a 20% strength by weight                                                 aqueous solution of ethoxyl-                                                  ated fatty alcohols (C.sub.16 -C.sub.18,                                      EO degree: 30)                                                          36    g of a 20% strength by weight                                                 aqueous solution of the                                                       surface active substance                                                      corresponding to Dowfax 2A1                                             172   g of water                                                              Stream II:                                                                    9.0   g of a 20% strength by weight                                                 aqueous solution of the                                                       surface active substance                                                      corresponding to Dowfax 2A1                                             15.0  g of SIa) (Example 1)                                                   100   g of water                                                              Stream III:                                                                   2.7  g of ascorbic acid                                                       0.09 g of iron (II) sulfate                                                   100  g of water                                                               ______________________________________                                    

Fo): As for Fn), except that in the initial charge the 256.94 g of Fm)were replaced with the corresponding amount of Fn).

Fp): As for Fn), except that in the initial charge the 256.94 g of Fm)were replaced with the corresponding amount of Fo). Additionally instream I the vinyl acetate was replaced with the corresponding amount ofstyrene and in stream III the amount of ascorbic acid was increased to5.4 g.

Fq): As for Fp), except that in the initial charge the 256.94 g of Fo)were replaced with the corresponding amount of Fp).

Fr): As for Fq), except that in the initial charge the 256.94 g of Fp)were replaced with the corresponding amount of Fq).

The final aqueous polymer dispersions thus obtained are characterized inTable 2 by the criteria set out for Table 1.

                                      TABLE 2                                     __________________________________________________________________________    SC     SV η                                                                             200                                                                              300 400                                                                              600                                                                              800 1000                                                                             1400                                        __________________________________________________________________________    Fh) 64.9                                                                             60.1                                                                             240 10 30  78 100                                                   Fi) 65.2                                                                             60.4                                                                             300 34 48  56 92 100                                                Fj) 64.8                                                                             60.0                                                                             200 22 46  73 83 95  100                                            Fk) 64.6                                                                             59.8                                                                             190 25 52  64 85 92  96 100                                         Fl) 64.7                                                                             59.9                                                                             150 16 28  70 100                                                   Fm) 62.4                                                                             57.8                                                                             83  16 50  65 83 83  95 100                                         Fn) 61.1                                                                             56.6                                                                             65  23 47  57 85 94  94 100                                         Fo) 60.1                                                                             54.7                                                                             35  28 46  65 78 92  98 100                                         Fp) 60.1                                                                             55.7                                                                             160 43 66  72 88 95  97 100                                         Fq) 60.6                                                                             55.6                                                                             87  28 64  82 92 98  100                                            Fr) 62.0                                                                             56.9                                                                             120 28 56  78 96 98  100                                            Fs):                                                                             A mixture consisting of                                                    83.33 g of SIIc (Example 2)                                                   2.00  g of sodium peroxodisulfate and                                         200   g of water                                                              __________________________________________________________________________

was heated to 85° C. and while that temperature was maintainedcontinuously admixed within 30 min with 4% by weight of stream I toproduce a starting dispersion IIf). Then without any time in between theaddition was commenced of the remainder of stream I and the total amountof stream II and carried out continuously and completed within 320 min.Finally, the mixture was postpolymerized at 85° C. for 1 h.

    ______________________________________                                        Stream I:                                                                     1710   g of n-butyl acrylate                                                  250    g of methyl methacrylate                                               40     g of methacrylic acid                                                  80     g of a 20% strength by weight aqueous solution                                of the surface active substance corresponding                                 to Dowfax 2A1                                                          50     g of a 20% strength by weight aqueous solution of                             ethoxylated fatty alcohols (C.sub.16 -C.sub.18,                               EO degree: 18)                                                         1      g of tert-dodecyl mercaptan                                            1      g of 3-mercaptopropyltrimethoxy-                                              silane                                                                 307    g of water                                                             Stream II:                                                                    8      g of a 25% strength by weight                                                 aqueous ammonia solution                                               13.33  g of SIa) of Example 1                                                 20.00  g of a 20% strength by weight aqueous solution                                of the surface active substance corresponding                                 to Dowfax 2A1                                                          8      g of sodium peroxodisulfate                                            100    g of water                                                             ______________________________________                                    

Ft): As for Fs), except that the 83.33 g of SIIc) in the initial chargewere replaced with 250 g of Fs) (Example 3) and that in stream I the 307g of water were replaced with 318 g of water.

Fu): As for Fs), except that the 83.33 g of SIIc) in the initial chargewere replaced with 166.7 g of SIIc) and that in stream I the 307 g ofwater were replaced with 292 g of water.

Fv): As for Fs), except that the 83.33 g of SIIc) in the initial chargewere replaced with 250 g of Fu) and that in stream I the 307 g of waterwere replaced with 318 g of water.

Fw): As for Fs), except that the 83.33 g of SIIc) in the initial chargewere replaced with 250 g of SIIc) and that in stream I the 307 g ofwater were replaced with 277 g of water.

Fx): As for Fs), except that the 83.33 g of SIIc) in the initial chargewere replaced with 250 g of Fw) and that in stream I the 307 g of waterwere replaced with 318 g of water.

The final aqueous polymer dispersions thus obtained are characterized inTable 3 according to the criteria set out for Table 1.

                                      TABLE 3                                     __________________________________________________________________________    SC    SV η                                                                            200                                                                              300                                                                              400 600                                                                              800                                                                              1000                                                                             1500                                                                             2000                                        __________________________________________________________________________    Fs)                                                                              72.4                                                                             66.5                                                                             240                                                                              3  5  40  52 57 61 100                                            Ft)                                                                              72.2                                                                             66.3                                                                             220                                                                              3  10 46  52 58 83 87 100                                         Fu)                                                                              72.5                                                                             66.6                                                                             360                                                                              2  25 40  44 47 88 100                                            Fv)                                                                              72.6                                                                             66.7                                                                             270                                                                              3  12 38  40 77 80 82 100                                         Fw)                                                                              72.4                                                                             66.5                                                                             420                                                                              4  36 38  40 58 97 100                                            Fx)                                                                              72.8                                                                             66.9                                                                             190                                                                              4  24 28  66 70 75 100                                            __________________________________________________________________________

We claim:
 1. A process for preparing a final aqueous polymer dispersionhaving a solids volume concentration of at least 50% by volume and apolydisperse particle size distribution, which comprises polymerizing atleast one radical polymerizable monomer with the addition of at leastone aqueous starting dispersion I of a starting polymer I and with theaddition of at least one aqueous starting dispersion II of a startingpolymer II in a polymerization vessel by the method of free radicalaqueous emulsion polymerization in the presence of surface activesubstances and free radical polymerization initiators with the provisothata) the starting polymer I is present in the at least one aqueousstarting dispersion I in the form of dispersed starting polymerparticles I whose nonzero weight average particle diameter is equal toor less than 50 nm, b) the mass of the at least one starting polymer Iadded relative to the total mass of the at least one radicalpolymerizable monomer and of the starting polymers I and II added in theform of aqueous starting dispersions I and II is from 0.2 to 0.7%, c)the starting polymer II is present in the at least one aqueous startingdispersion II in the form of dispersed starting polymer particles II ofwhose total mass not more than 60% have a non-zero particle diameter≦200nm and at least 40% have a particle diameter>200 nm, d) the mass of theat least one starting polymer II added relative to the total mass of theat least one radical polymerizable monomer and of the starting polymersI and II added in the form of aqueous starting dispersions I and II isfrom 2 to 20%, e) of the total amount of the at least one aqueousstarting dispersion I to be added the amount introduced into thepolymerization vessel as initial charge prior to the start of the freeradical aqueous emulsion polymerization is nil, f) the total amount ofthe at least one aqueous starting dispersion II to be added isintroduced into the polymerization vessel as initial charge prior to thestart of the free radical emulsion polymerization, g) of the totalamount of the at least one monomer to be polymerized the proportionintroduced into the polymerization vessel as initial charge prior to thestart of the free radical aqueous emulsion polymerization is nil, h) thetotal amount of the at least one monomer to be polymerized is added tothe polymerization vessel from the start of the free radical aqueousemulsion polymerization in such a way that at any time of this additionthe polymerization conversion of the total monomers already addedpreviously to the polymerization vessel is at least 80 mol %, i) at anytime after the start of the free radical aqueous emulsion polymerizationthe ratio V of the number of moles of the particles of starting polymerI already added previously to the polymerization vessel in the form ofthe at least one aqueous starting dispersion I to the number of moles ofthe amount already added previously to the polymerization vessel of theat least one monomer to be radical polymerized, standardized for theratio of the number of moles of the total starting polymer particles tobe added in the form of the at least one aqueous starting dispersion Ito the number of moles of the total amount of monomers to be radicalpolymerized, is within the range from >0 to 10, j) on completion of theaddition of the total amount to be added of the at least one aqueousstarting dispersion I additionally at most 20 mol % of the total amountof the at least one monomer to be radical polymerized is added to thepolymerization vessel, k) the at least one aqueous starting dispersion Ito be added contains at least 10-50% by weight, based on its startingpolymer I content, of surface active substances, l) the at least oneaqueous starting dispersion II to be added contains at least 1-5% byweight, based on its starting polymer II content, of surface activesubstances, m) the final aqueous polymer dispersion contains from 1 to5% by weight of the surface active substances, based on the total massformed of the at least one radical polymerizable monomer and of thestarting polymers I and II to be added in the form of aqueous startingdispersions I and II, n) the amount of surface active substancescontained in the polymerization vessel at any time from the start of thefree radical aqueous emulsion polymerization minus the minimum amountsmentioned under k) and l) of surface active substances introduced intothe polymerization vessel is less than 5% by weight, based on themonomers already added previously to the polymerization vessel, and o)the total amount used of free radical polymerization initiators is from0.1 to 5% by weight, based on the total amount of the at least onemonomer to be radical polymerized, and is added to the polymerizationvessel in the course of the free radical aqueous emulsion polymerizationin such a way that the free radical aqueous emulsion polymerizationcontinues to a minimum polymerization conversion of the total amount ofthe at least one monomer to be polymerized of at least 90 mol %.
 2. Theprocess as claimed in claim 1, wherein the at least one aqueous startingdispersion II used is a final aqueous polymer dispersion obtained fromthe process of claim
 1. 3. The process as claimed in claim 1, whereinthe at least one aqueous starting dispersion II used is a final aqueouspolymer dispersion obtained by the process of claim 1, at least part ofthe final aqueous polymer dispersion obtained therein is in turn used asat least one aqueous starting dispersion II in a subsequent cycle toobtain a subsequent final polymer dispersion using the process of claim1, and the cyclical method thus delineated is continued as desired.
 4. Aprocess as claimed in claim 2, wherein a first final polymer dispersionin a polymerization vessel is only part-discharged and the remainder isused with or without a dilution as at least one starting dispersion IIin a subsequent process as claimed in claim 2 and the cyclical methodthus delineated is continued as desired.
 5. The process as claimed inclaim 1, wherein the at least one aqueous starting dispersion II usedhas the following particle size distribution:5-25% by weight of startingpolymer II≦200 nm 15-40% by weight of starting polymer II≦400 nm 35-75%by weight of starting polymer II≦600 nm 45-90% by weight of startingpolymer II≦1600 nm.
 6. The process as claimed in claim 1, wherein thestarting polymer I is present in the at least one aqueous startingdispersion I in the form of dispersed starting polymer particles whoseweight average particle diameter is within the range from 25 to 45 nm.7. The process as claimed in claim 1, wherein the monomers to bepolymerized are added to the reaction zone in such a way that at anytime of this addition the polymerization conversion of the monomersalready added previously to the polymerization vessel is at least 90 mol%.
 8. The process as claimed in claim 1, wherein the monomers to bepolymerized and the at least one aqueous starting dispersion I to beadded are added continuously and synchronously starting at the sametime.
 9. The process as claimed in claim 1, wherein the final aqueouspolymer dispersion comprises a polymer comprisingfrom 70 to 100% byweight of styrene or esters of acrylic or methacrylic acid, or a mixturethereof, with alkanols of from 1 to 12 carbon atoms, or a mixturethereofor from 70 to 100% by weight of butadiene or a mixture ofbutadiene and styreneor from 70 to 100% by weight of vinyl chloride orvinylidene chloride or a mixture thereof.
 10. The process as claimed inclaim 1, wherein said surface active substances comprise a surfaceactive substance of the formula I ##STR2## where R¹ and R² are eachhydrogen or C₄ -C₂₄ -alkyl but are not both hydrogen, and X and Y areeach an alkali metal or ammonium ion.
 11. The process as claimed inclaim 1, wherein the final aqueous polymer dispersion obtained has asolids volume concentration of ≧65% by volume.